COI Funded Project: Implementation of an Atmospheric Mesoscale Model in the Gulf of Maine

Proposed Research

Modeling or forecasting coastal ocean dynamics requires accurate synoptic
estimates of atmospheric forcing fields. We will implement and evaluate
a high resolution regional atmospheric model focused in the Gulf of
Maine. The model uses moderate resolution model data as initial conditions
and is capable of assimilating near real-time buoy and ship measurements.
The resulting implementation will provide a framework for improving
coastal ocean models and subsequently enhance research in coastal
physical and biological dynamics.

Final Report

Introduction
Oceanic variability on the continental shelf is closely tied to
temporal and spatial variability in atmospheric forcing. To accurately
model or forecast the coastal ocean response to this forcing, synoptic
estimates of air-sea heat, freshwater and momentum fluxes are required
as surface boundary conditions. No observing system yet exists to
provide all of these flux measurements, so ocean modelers often
rely on the surface fields from Numerical Weather Prediction (NWP)
models as realistic surface forcing input. The U.S. National Centers
for Environmental Prediction (NCEP) run both global and regional
NWP models from which surface fields are available for use in ocean
research. The NCEP NWP regional model fields are attractive for
use in coastal ocean modeling because they can provide near real-time
gridded estimates of the air-sea fluxes at relatively high spatial
and temporal resolutions over much of the U.S. continental shelf.

The use of regional NWP products
for synoptic meteorological or air-sea flux fields is sure to increase
as interest in the coastal zone continues to grow. While the regional
NWP model surface fields have great potential for ocean research,
use of these fields must be accompanied by an understanding of their
errors.

We used the fifth Fifth-Generation
NCAR / Penn State Mesoscale Model (MM5) was originally developed
at Penn State nearly 30 years ago. MM5 is a community model designed
for analysis of limited areas requiring higher resolution. The current
version is a state of the art model that includes nonhydrostatic
dynamics, data assimilation, nested grids and parallel optimizations.
The MM5 model requires the output from a large-scale atmospheric
model for initialization and lateral boundary conditions. The model
uses continuous four-dimensional data assimilation, which will enable
the use of in situ measurements to improve the meteorological
forecast. The high-resolution model analysis combined with in
situ measurements will provide a mechanism to study weakly
forced or rapid events that are typically not captured in operational
forecasts.

Our goal was to acquire and
implement a research quality numerical weather forecast system to
produce better and higher resolution forcing fields in the Mid-Atlantic
Bight south of Cape Cod . This allowed us to ascertain the strengths
and weaknesses of numerical weather prediction data, particularly
over the ocean where in situ boundary conditions are sparse.
The resulting implementation provides a framework for improving
coastal ocean models and subsequently enhanced research in coastal
physical and biological dynamics. The model is currently being run
in a 24 hour forecast mode using the two nested domains described
below.

Model Implementation
We acquired the MM5 model
from the National Center for Atmospheric Research (NCAR), built
and ran the Storm of the Century test case on an SGI workstation.
This test case was used to exercise the various components of the
model and verify the results against simulations run at NCAR.

The model requires a number
of pre-processing routines to be executed prior to the model forecast.
The first program is the terrain program and is only run once. Terrain
sets up the elevation and land use characteristics for the model
grids. We used the US Geological Survey’s 30-second (0.925 km) data
for our analysis. The second program is the regrid program and is
comprised of a suite of programs called pregrid and an interpolation
program called regridder . The pregrid routines ingest data from
a variety of large-scale models and produce an intermediate format
suitable for the regridder program. The regridder program reads
this intermediate file and produces gridded data suitable for the
MM5 modeling system. The third program is rawins and takes the output
of regridder and improves the first guess using observations. The
final pre-processing routine, interpf , takes this data and interpolates
it in a consistent manner onto the model grid. When these procedures
are complete, the model is run.

The entire process was automated
on a two processor Linux workstation at WHOI. The latest National
Centers for Environmental Prediction (NCEP) 32 km ETA forecast is
downloaded every night. This forecast is used as initial and boundary
conditions for the nested model we have implemented here. A 24-hour
model run requires about 90 minutes of CPU time to complete. The
model results and the NCEP input data are then archived.

Several software packages
were available for plotting the results of the MM5 model runs. However,
since these were oriented toward atmospheric analysis, a program
was written to extract surface variables from the model output.
We are able to use this output with existing analysis and plotting
programs to study the results.

Model
Domains
The
model was tested on the east coast with two nested grids focusing
on the Mid-Atlantic Bight southwest of Martha's Vineyard . The initial
nested grid is shown in Figure 1. The red dots show the regional
domain at 27 km resolution extending from Chesapeake Bay through
the Gulf of Maine . Figure 2 shows a closer view the first nested
grid at 9 km covering the Mid-Atlantic Bight. The 3 km fine resolution
grid centered near Martha’s Vineyard is shown in Figure 3.

Results
The Mid-Atlantic Bight
MM5 model is currently run in a semi-operational mode and produces
a 24-hour forecast with output every 3 hours at each of the 3 grid
resolutions. This is primarily dependent upon the availability of
the workstation resources combined with the network accessibility
of the NCEP ETA input data. We are currently capable of supplementing
research programs needing synoptic views of small-scale atmospheric
features anywhere around the globe. Our work has facilitated analysis
of an MM5 model run at NCAR for NSF GLOBEC sponsored research in
the Southern Ocean.

Typical results from the
model run on August 18, 2003 are shown in Figures 4-6. Figure 4
shows the large-scale grid surface air temperature overlaid with
surface wind vectors. The forecast was initiated on August 18, 2003
at 00:00 GMT and the forecast time was August 18, 2003 at 09:00
GMT.

The 9 km grid is shown
in Figure 5. The increased resolution can be seen in the delineation
of the surface temperature at the coast.

The 3 km fine resolution
grid is shown in Figure 6. At this resolution, the land sea temperature
differences are apparent. At this resolution, surface fluxes can
be compared with near shore in situ observations at the WHOI Martha’s
Vineyard Coastal Observatory to improve model physics at the surface
boundary layer and provide adequate input for coastal ocean models.

The mid-Atlantic bight region
MM5 has been run in support of CBLAST and other ongoing operations
in the region. The model was run nearly continuously from
June 2001 through June 2002 due to hardware failures. Three-hours
surface flux fields have been archived and can be made available
for retrospective studies. Acquisition of new hardware enabled
the continuation of the MM5 system in April 2003 and it is still
operational today. There are no immediate plans to stop the
system which will continue to operate largely unattended as long
as hardware and software configurations remain stable.

Conclusions
We have acquired and implemented
a semi-operational mesoscale atmospheric model in the Mid-Atlantic
Bight. We have advanced our knowledge of the strengths and limitations
inherent in mesoscale models and handling the large data sets associated
with those models. This knowledge has facilitated ongoing research
based in other geographic regions such as the Southern Ocean.
In addition, it has enabled us to develop competitive proposals
to use MM5 and other model derived surface fluxes in support of
air-sea interaction research within WHOI.

Originally published: January 25, 2000

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